It is often desirable to detect when an airflow path becomes detrimentally obstructed, such as when a filter becomes clogged or a heat exchanger becomes clogged or freezes over.
Modem automotive and other heat exchangers and air conditioners use electric fans to accelerate the airflow. A fan moving air through a clogged airflow path may suffer a short life and lead to engine damage and/or increased vehicle maintenance costs. Because such fans are often located in hard to reach locations, a failed fan for an automobile air conditioner or cooling system can significantly increase repair costs. It is therefore useful to detect and correct airflow obstructions when such problems occur and before repairs are required. The problem of determining when an airflow path becomes detrimentally obstructed has recently been addressed by placing a sensor in the air flow path and detecting when the airflow rate decreases. Sensors in previous apparatus or this purpose include electrical current sensors (a heavily loaded fan draws additional current), vane switches, pressure switches, and the like.
It is known from U.S. Pat. No. 4,479,115 to Holzhauer to provide a mechanism for automatically determining the speed of a fan, such as a fan used to cool electronic equipment, and/or to determine when the fan has failed, i.e., slowed down or stopped. Holzhauer discloses that one possible approach is to monitor the fan speed by sensing the rate at which an optical path is interrupted by the fan blades. Holzhauer also discloses that it is possible to monitor the fan speed by using a Hall effect device (such as a Hall Effect Transistor or "HET") to sense the rate at which the magnetic field created by the fan motor is rotating.
However, Holzhauer points to a significant disadvantage of the Hall effect device sensors, namely, that the HET must be installed when the fan is manufactured. The relatively heavy magnet required to trigger the HET unbalances the fan blade when added near an external HET sensor. Thus, such an arrangement is not readily retrofit or incorporated into existing installations. That is, it must be an integral part of the initial fan design and cannot, therefore, be conveniently added to existing fans or readily incorporated into existing fan designs. Moreover, stray magnetic fields are well-known to negatively affect the operation of HET sensors. The electric fan motor, of course, generates strong interfering magnetic fields in normal installations unless it is shielded.
In the typical HET, a Hall element, including a plurality of semiconductor materials of different conductivity types, is subjected to a changing magnetic field so as to deflect charge carriers in the Hall element produced by passing a current therethrough. The HET device measures the extent of charge carrier deflection and thus the flux density in terms of a variable voltage appearing between terminals at opposite ends of the Hall element. Such voltage is generally proportional to the flux density through the Hall element, and may therefore be used to measure or otherwise represent a number of different parameters which can be translated into a magnetic field. U.S. Pat. No. 3,835,373 to Matula teaches that a principal difficulty in the use of HET's to measure parameters arises because the magnetic flux density varies hyperbolically in a proportional air gap producing a non-linear relation between position and Hall element output voltage. This nonlinear relationship is highly undesirable in many measurement applications.
The Matula patent discloses a complex rotational positional sensor which utilizes a HET and a means to maintain the Hall voltage constant. The rotational position sensor includes a HET which is mounted in an air gap of variable size within a magnetic circuit. The HET is subjected to changes in magnetic flux density in direct relation to the rotational position of a cylinder member forming a part of the magnetic circuit. The magnetic circuit includes a C-shaped permanent magnet having opposite pole pieces forming air gaps with the cylindrical member. The cylindrical member comprises a half cylinder in the region of the air gap containing the HET so as to vary the effective area of the gap and thus the flux density as the cylindrical member turns. Such a system is very complex and also cannot be readily incorporated or retrofit into pre-existing fans.
U.S. Pat. No. 4,524,932 to Bodziak describes a railroad car wheel detector which utilizes a Hall effect element. The HET is incorporated into a complex integrated circuit package including temperature compensation, voltage regulation, and amplification functions. It is mounted on top of a permanent magnet which is made of ceramic material with the critical Hall axis aligned with the magnet pole axis. The combined permanent magnet and HET assembly is mounted on the rail at a predetermined distance below the top of the rails so that the flange of each passing wheel occupies the air gap between the magnet and the rail through which the major portion of the magnetic flux flows. Reduction of the air gap increases the level of the magnet flux and thus the level of voltage output of the Hall element.
U.S. Pat. No. 4,719,419 to Dawley discloses that one known apparatus for sensing precise shaft position utilizes an annular ring magnet having a plurality of circumferentially oriented poles. The ring magnet is coaxially attached to a rotary shaft and a Hall effect device is attached to a stationary member adjacent the ring magnet. In particular, the ring magnet includes a plurality of magnets connected in series to form a ring with the north and south poles of the magnets alternately arranged. As the shaft and the attached ring magnet rotate, the Hall effect device generates a sinusoidal electrical signal indicative of the magnetic induction or the magnetic flux density produced by the magnets at the Hall effect device. The polarity of the magnetic flux density and the generated electrical signal changes as each pole passes the Hall effect device. The number of signal cycles per revolution of the ring is a function of the number of poles that make up the ring. A counter counts the number of waveform cycles of the signal generated by the Hall effect device. The count is then used to determine the rotary position of the shaft. The accuracy of such a system is dependent upon the number of poles that make up the ring. That is, increasing the angular position accuracy is accomplished by increasing the number of magnets. Such a system is very complex and also cannot be readily incorporated or retrofit into pre-existing fan designs and fans.